In the recent trend toward further reduction in weight and enhancement in power of automobiles, valve springs for automobile engine, suspension springs for suspension, clutch springs, brake springs or the like are requested to be designed for withstanding higher stress. Namely, there is a demand for springs excellent in sag resistance and fatigue property, along with the increase in loaded stress on the springs.
Sag resistance is known to be improved by strengthening of spring material. Because sag resistance is improved by strengthening, for example, by adding Si in a greater amount, Si is normally used in an amount in the range of approximately 0.8 to 2.5% (Japanese Patent No. 2898472, Japanese unexamined patent publication No. 2000-169937 and others). In addition, strengthening of spring material would be effective in improving fatigue property, from the point of fatigue limit. However, when a spring material is strengthened, the defect sensitivity of the spring tends to be high, and that sometimes makes the fatigue life of the spring shorten and often involves breakage of the spring during coiling. Thus, it is quite difficult to improve both sag resistance and fatigue property at the same time.
An object of the present invention, which was made under the circumstances above, is to provide a steel useful in producing springs capable of improving both sag resistance and fatigue property.